1. Field of the Invention
This invention relates to electronic packaging and more particularly to a method of making electronic packages which require a thin flexible printed circuit polyimide substrate of substantially five microns.
2. Description of the Prior Art
U.S. Pat. No. 3,780,352, FIG. 4, is an example of an electronic package assembly in which a thin sheet of insulating material, which supports thin metallic film strips, is positioned intermediate between a semiconductor device mounted thereon and a pin carrier of ceramic. The metallic film strips are bonded to the heads of the pins of the carrier and the contact pads of the semiconductor device.
Also, in the prior art of which we are aware, the formation of thin flexible polyimide printed circuit (PC) tapes have been generally heretofore limited to thicknesses of 0.5 to 3 mils, cf. U.S. Pat. No. 3,781,596, for example.
Because of the thermal mismatch between the pin carrier, PC member and/or semiconductor device, the bonds between the PC member and the pads of the semiconductor device and/or the heads of the pins are subject to stresses which cause failure of the bond joint and, hence, failure of the packaged device. These stresses may be caused, for example, by thermal cycling such as that encountered under a test environment used to test the packages or under actual environmental conditions during the operation of the package when installed in service in some circuit application.
It has been suggested that the PC member itself act as a strain relief to relieve the aforesaid stresses. However, the thicknesses of the prior art members were not amenable to providing the degree of flexibility required to accomplish the desired result. Moreover, the temperatures and/or pressures associated with the making of the PC member and/or the bonding at the IC pad sites and/or at the pin sites would stiffen the polyimide material at the vicinity of the sites. Thus, the flexibility of the polyimide material at the bonding sites was destroyed or lost so that it could not effect a satisfactory strain relief. With the advent of high density (i.e. 100 or more) input/output (I/O) terminal pad devices, the problem became even more acute. Moreover, the problem is even further compounded when the chip is rigidly affixed to another structural member such as is the case, for example, when the chip is backbonded to the cover of the assembly or an intervening thermal conductive member for enhancing the heat sinking, i.e. dissipation, of the heat associated with the chip and/or assembly.